Commit f44699a8 authored by Wanpeng Li's avatar Wanpeng Li Committed by Ben Hutchings

KVM: cpuid: Fix read/write out-of-bounds vulnerability in cpuid emulation

commit a3641631 upstream.

If "i" is the last element in the vcpu->arch.cpuid_entries[] array, it
potentially can be exploited the vulnerability. this will out-of-bounds
read and write.  Luckily, the effect is small:

	/* when no next entry is found, the current entry[i] is reselected */
	for (j = i + 1; ; j = (j + 1) % nent) {
		struct kvm_cpuid_entry2 *ej = &vcpu->arch.cpuid_entries[j];
		if (ej->function == e->function) {

It reads ej->maxphyaddr, which is user controlled.  However...

			ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT;

After cpuid_entries there is

	int maxphyaddr;
	struct x86_emulate_ctxt emulate_ctxt;  /* 16-byte aligned */

So we have:

- cpuid_entries at offset 1B50 (6992)
- maxphyaddr at offset 27D0 (6992 + 3200 = 10192)
- padding at 27D4...27DF
- emulate_ctxt at 27E0

And it writes in the padding.  Pfew, writing the ops field of emulate_ctxt
would have been much worse.

This patch fixes it by modding the index to avoid the out-of-bounds
access. Worst case, i == j and ej->function == e->function,
the loop can bail out.
Reported-by: default avatarMoguofang <moguofang@huawei.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Guofang Mo <moguofang@huawei.com>
Signed-off-by: default avatarWanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: default avatarBen Hutchings <ben@decadent.org.uk>
parent 0fbe3f35
......@@ -671,18 +671,20 @@ int kvm_dev_ioctl_get_cpuid(struct kvm_cpuid2 *cpuid,
static int move_to_next_stateful_cpuid_entry(struct kvm_vcpu *vcpu, int i)
{
struct kvm_cpuid_entry2 *e = &vcpu->arch.cpuid_entries[i];
int j, nent = vcpu->arch.cpuid_nent;
struct kvm_cpuid_entry2 *ej;
int j = i;
int nent = vcpu->arch.cpuid_nent;
e->flags &= ~KVM_CPUID_FLAG_STATE_READ_NEXT;
/* when no next entry is found, the current entry[i] is reselected */
for (j = i + 1; ; j = (j + 1) % nent) {
struct kvm_cpuid_entry2 *ej = &vcpu->arch.cpuid_entries[j];
if (ej->function == e->function) {
ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT;
return j;
}
}
return 0; /* silence gcc, even though control never reaches here */
do {
j = (j + 1) % nent;
ej = &vcpu->arch.cpuid_entries[j];
} while (ej->function != e->function);
ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT;
return j;
}
/* find an entry with matching function, matching index (if needed), and that
......
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